Out in the cold: Royal Bank of Scotland is one of the quarter of FTSE-100 companies with no women on their executive committees. Photograph: Andrew Winning/Reuters

One of the City's biggest fund managers – Legal & General – has warned that it will vote against the chairman of any FTSE 100 company that persists with an all-male board after the government's two-year deadline for board diversity.

The fund-management group is in the process of rewriting its policies for annual meetings and is telling companies that from 2015 it will vote against the chairman or the chairman of the nominations committee if they have not installed any women on the board, or if aspirational targets have not been set, or if disclosure about gender balance is inadequate.

L&G has been considering the move for a number years, and particularly in light of Lord Davies's recommendations that a quarter of boardroom positions should be held by women by 2015.

Figures published by the government this week showed that the Davies targets – set in 2011 – have resulted in more female non-executive directors (part-time directors who are not company employees and who provide outside expertise and oversight). Women now hold almost 24% of non-executive seats and only five out of the top 100 companies are now men-only domains.

But the progress of women in full-time executive roles has been slower. Women hold just 6.1% of executive positions, compared with 5.5% when Davies reported.

Earlier this week the business secretary, Vince Cable, said it was crucial to focus on the management level just below the main board – usually called the executive committee – to get more women into top jobs. However, an analysis of FTSE-100 companies by the Guardian shows that one in four of the UK's largest listed firms have no women on their executive committees. Overall, just 14.7% of places on these committees are occupied by women.

A similar exercise conducted earlier this year by Cranfield Business School found the proportion of women on executive committees has fallen from 17.2% in 2010 to 15.3% in 2013.

One of the all-male executive committees is at Royal Bank of Scotland, the 81% taxpayer-owned bank. Chris Sullivan, chief executive of RBS's corporate banking division who sits on the committee, said at a conference this week that he did not believe in quotas to ensure more women are promoted.

"I am 150% against quotas because the consequences can be horrific. You can hit your quota with unqualified people and then people say that gender equality doesn't work," Sullivan said.

The coalition government also opposed proposals to introduce mandatory quotas when they were discussed by the European commission, although it has been vocal on the topic of boardroom diversity.

Yesterday Cable told the Guardian: "More women in the boardroom is an important first step, but our efforts must also extend into the executive level of companies as well.

"Too often we are seeing talented women leave their jobs in the middle parts of their career to look after children and not return to work. This not only creates a brain drain in key sectors, but it is also hindering women's career progression because they are playing catch-up with their male counterparts if they do return.

"These women are the CEOs of tomorrow, and we cannot look to build a stronger economy without tapping into the full talent of our workforce."

The problem of companies losing female employees is most acute for women aged between 30 and 35, although attrition rates are notable through all stages of female careers. A report published last year by the City headhunting firm MWM Consultin g on getting more women into senior executive roles found that women represented 38% of lower or middle management roles, but only 11% at executive committee level.

Michael Reyner, a partner at MWM, said: "At entry level companies employ roughly half men and half women. But the numbers of women then decreases right the way through a company. The problem is not so much a 'glass ceiling' as often suggested, but much more of a 'slippery ladder.'"

The issue of the executive pipeline was one of the main discussion points at by the Women's Business Forum and fronted by diversity consultant Heather Jackson, included speeches from leading businessmen. The second was held by the Chartered Management Institute and included contributions from Susan Vinnicombe, a professor at Cranfield School of Management and Helen Calcraft, the co-founder of the advertising agency Lucky Generals.

Jackson said: "I'd like to apologise to the next generation, because too many of my peers and friends packed their careers in because they didn't like the culture rather than working to try and change the organisation."